Comparison of five methods to determine the cation exchange capacity of soil

Abstract

AbstractBackgroundCation exchange capacity (CEC) is a routinely measured soil fertility indicator. The standard NH4OAc (pH 7) extraction procedure is time‐consuming and overestimates actual CEC values of variable charge soils. Unbuffered extractants have been developed to measure the effective CEC (eCEC), but they differ in the type of index cation and extraction procedures.AimThis study was set up to systematically compare CEC values and exchangeable cation concentrations among different procedures and evaluate their practical aspects.MethodsFive procedures were compared for (e)CEC, that is, silver thiourea (AgTU), cobalt(III) hexamine (Cohex), compulsive exchange (CE, i.e., BaCl2/MgSO4), BaCl2 (sum of cations in single‐extract), and NH4OAc (pH 7). We applied these methods to a set of 25 samples of clay minerals, peat, or samples from soils with contrasting properties.ResultsThe CEC values correlated well among methods (R2 = 0.92–0.98). Median ratios of eCEC (AgTU as well as CE) to the corresponding eCEC (Cohex) value were 1.0, showing good agreement between eCEC methods, but NH4OAc exceeded Cohex values (ratios up to 2.5 in acid soil). For BaCl2‐extracteable cations, the ratio ranged from low (<1.0) in acid soils (acid cations not measured) to high (>1.0) in high‐pH soil (dissolution of carbonates). Multiple‐extraction methods (CE and NH4OAc) yielded more variation and increased labor.ConclusionsThe chemical properties of the sample cause method‐specific interactions with chemical components of extractants. We found the Cohex method with ICP‐MS detection to be the most efficient and cost‐effective technique for determination of eCEC and exchangeable cations.